11.1.1 Sciatic Plexus

The sciatic plexus derives its roots from part of the anterior ramus of the fourth lumbar nerve and from the entire anterior ramus of the fifth lumbar nerve, which together form the lumbosacral trunk, along with the anterior rami of the first and second and part of the third sacral nerves. The anterior ramus of the first sacral nerve is not only the biggest branch of the lumbosacral plexus but the biggest anterior ramus overall.

All of the roots of the plexus converge from their exit sites toward the greater sciatic foramen so that the plexus forms a triangular sheet, the apex of which points toward the infrapiriform foramen where the sciatic nerve emerges (Fig. 11.1). The nerve plexus lies for the most part on the piriformis muscle and is covered toward the pelvis by the parietal peritoneum and the tissues lying below it that form the parietal pelvic fascia and branches of the internal iliac artery. The articular branches that supply parts of the hip joint capsule and the periosteal branches that innervate the periosteum of the sciatic tuber, greater trochanter, and lesser trochanter also come from the sciatic plexus.

11.1.2 Sciatic Nerve (L4–S3)

The sciatic nerve derives its fibers from all the roots of the sacral plexus. It is the biggest and longest nerve in the body, it supplies the widest area, and at the same time it has the greatest resistance among all the nerve cords, with a tear strength of 91.5 kg (!). Excessive stretching can even tear the nerve trunk from its roots in the spinal cord. The roots of the sciatic nerve unite into the trunk immediately before the greater sciatic foramen at the lower border of the piriformis (Fig. 11.2).

The sciatic nerve consists of two components, the common fibular nerve (synonym: common peroneal nerve) and the tibial nerve, which are surrounded in the lesser pelvis and thigh by a shared connective-tissue sheath and therefore have the appearance of a single nerve trunk. The division into the two branches can occur at varying levels (Fig. 11.3 and Fig. 11.4).

At dissection, the nerve can almost always be separated into its two divisions as far as the hip region, even if they run in a common sheath (Ericksen et al 2002).

The sciatic nerve usually leaves the true pelvis (lesser pelvis) through the infrapiriform foramen as a 1.4 cm (up to 3 cm) wide and 0.4 to 0.5 cm (up to 0.9 cm) thick nerve cord (Fig. 11.3 and Fig. 11.4) and enters the gluteal region. It divides into the tibial nerve and the common fibular nerve at the latest where it enters the popliteal fossa.

The sciatic nerve provides motor innervation through its tibial division to all of the flexor muscles of the thigh (with the exception of the short head of the biceps femoris) and the lower leg and with its fibular division it innervates the short head of the biceps femoris and the fibular muscles and all the extensors in the lower leg and foot. It provides sensory innervation through both divisions to the skin of the lower leg and foot.

11.1.3 Posterior Cutaneous Nerve of the Thigh (S1–S3)

The posterior cutaneous nerve of the thigh, which is the sensory supply to the posterior aspect of the thigh, leaves the pelvis together with the sciatic nerve and the inferior gluteal nerve through the infrapiriform foramen. The nerve lies medial to the sciatic nerve and reaches the posterior surface of the thigh under the gluteus maximus muscle. There it lies subfascially, but close to the fascia lata, through which it passes at varying levels. The distribution of the posterior cutaneous nerve of the thigh is variable and extends from the distal third of the buttocks as far as the distal boundary of the popliteal fossa.

11.1.4 Periosteal Innervation

11.2.1 Technique of Anterior Sciatic Nerve Block

The classical technique of anterior sciatic nerve block was described in 1963 by G.P. Beck (Beck 1963). The technique was modified by Meier (Meier 1998) to simplify the procedure and allow placement of a catheter.

Anterior Block of the Sciatic Nerve (according to Beck, Classical Technique)

Landmarks

Inguinal crease, gap between the rectus femoris and sartorius muscles. (Note: We have dispensed with the extensive description of orientation with various landmarks and lines, as the method described below has been proven for years.)

Position

The patient lies supine with the leg to be anesthetized extended.

Procedure

The puncture site is approximately 10 cm (hand breadth) below the inguinal crease (Fig. 11.5).

The puncture site is the muscle gap between the rectus femoris and the sartorius. In this muscle gap, vertical pressure is exerted on the femur with two fingers, and the bone is used as an abutment (“two-finger grasp”; Fig. 11.6, Fig. 11.7, Fig. 11.8). The blood vessels are pushed medially by this maneuver and the likelihood of accidental vascular puncture is reduced (Fig. 11.7, Fig. 11.8, Fig. 11.9, Fig. 11.10). A 15-cm needle is then advanced at an angle of 75 to 85° to the skin in cranial, posterior and slightly lateral direction (Fig. 11.8 and Fig. 11.10). When a cranial direction is maintained, the sciatic nerve is reached further proximal by a few centimeters, depending on the angle of insertion; if the nerve is reached more tangentially, it is easier to advance an indwelling catheter (Fig. 11.11). For ultrasound visualization, a curved array transducer is recommended owing to the depth (Fig. 11.12, Chapter 11.2.5). The sciatic nerve can be reached well using this technique.

Branches of the femoral nerve are often stimulated after the needle is advanced a few centimeters. The position of the needle tip is corrected (usually laterally) until no further response can be detected from the quadriceps femoris muscle and it is then advanced further. Stimulation is initially with a current of 0.8 to 1.0 mA. After 6 to 10 cm, the adductor fascia is reached, which is often signaled by an obvious loss of resistance. The needle is advanced further until a motor response is produced in one of the two divisions of the sciatic nerve (fibular nerve, dorsiflexion: tibial nerve, plantar flexion; Fig. 11.13).

Practical Note

The correct position of the needle is indicated by a response in the foot. If there is a response in the ischiocrural muscles, the needle must be withdrawn significantly and corrected laterally (“below the femur”) (Fig. 11.14, Fig. 11.15, Fig. 11.16). Then 30 mL of a medium-acting or long-acting local anesthetic is injected.

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  In the continuous technique, following injection of the local anesthetic, a 20G catheter is advanced 4 cm proximally by the needle (Fig. 11.17 and Fig. 11.18). Short, mild resistance can occur when the catheter tip reaches the end of the needle (Fig. 11.17), but this is normally easily overcome, and the catheter can then be advanced smoothly.

  
  
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  The technique should be performed with peripheral nerve stimulation.

  
  
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  If sciatic nerve block is combined with a femoral nerve block, anesthetizing the femoral nerve first is useful as this leads to anesthesia of the anterior thigh and helps to improve patient comfort (Fig. 11.17).

  
  
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  The technique can also be performed in obese patients (Fig. 11.19 and Fig. 11.20).

11.2.2 Indications and Contraindications (in Combination with Femoral Nerve Block)

Indications

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  Operations on the knee, lower leg, or foot (e.g., knee replacement, tibial head osteotomy, arthrodesis, lateral ligament suture, forefoot operation).

  
  
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  Reposition of fractures in the lower leg and foot area.

  
  
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  Amputations in the thigh (Fig. 11.21), lower leg, and foot.

  
  
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  Regional sympathetic block (e.g., perfusion disorders, wound healing disorders, CRPS 1).

  
  
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  Pain therapy (e.g., postoperative, achillodynia, oligoarthritis).

  
  
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  Postoperative pain therapy after total knee replacement, particularly when there is incomplete extension in the knee joint (Fig. 11.22).

  
  
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  Traumatology (e.g., pain-free positioning for diagnostic investigation).

11.2.3 Side Effects and Complications

There are no known special complications of sciatic nerve block. There have been few reports of side effects. Major complications including late sequelae are very rare. Dysesthesia for 1 to 3 days, which resolved spontaneously, has been described.

11.2.4 Remarks on the Technique

In Beck′s classical anterior technique (Beck 1963) the sciatic nerve is reached relatively distally. For this reason, anesthesia of the posterior cutaneous nerve of the thigh is often inadequate, which may cause problems in patients requiring a tourniquet at the thigh. In addition, with the Beck technique the sciatic nerve may be difficult to locate. With the modified technique, directing the needle at an angle of 75° to the skin causes needle contact with the sciatic nerve 3 to 4 cm more proximally; therefore both the sciatic nerve and the posterior cutaneous nerve of the thigh are reached (Meier 1999a).

The diameter of the sciatic nerve, the largest nerve in the body, is impressive. In anatomical studies only part of the nerve was stained after injection of 10 mL of methylene blue through the catheter (Meier 1999a; Fig. 11.23).

The distance from the skin of the anterior thigh to the sciatic nerve is around 6 to 10 cm in adults when the needle is directed vertically. With the needle directed cranially, the nerve is reached after 8 to 12 cm, and sometimes only after 13 to 15 cm in muscular or obese patients (Brown 1996, Chelly et al 1997, Bridenbaugh and Wedel 1998, Ericksen et al 2002). Paresthesia should be avoided.

During stimulation, note that contractions of gluteus maximus or tensor fasciae latae do not represent an adequate response. The vicinity of the sciatic nerve is indicated by a response in the area supplied by the nerve (hamstring muscles, triceps surae, tibialis anterior, fibular group; Wagner and Missler 1987, Kaiser et al 1990, Wagner 1994).

The motor response should be optimized so that plantar flexion (tibial nerve) or dorsiflexion (common fibular nerve) is produced in the foot.

In a study by Neuburger et al (2001), the stimulated muscle group (whether innervated by the tibial nerve or fibular nerve) had no effect on the block result. With correct stimulation (below 0.5 mA), a success rate over 95% can be achieved (Niesel 1994, Chelly et al 1999).

For anesthesia, 30 mL of a medium-acting or long-acting local anesthetic of adequate concentration should be injected (Wagner and Taeger 1988, Bridenbaugh and Wedel 1998, Chelly and Delaunay 1999; Fig. 11.25). In combination with a lumbar plexus block (psoas or so-called “3-in-1 block”), a further 30 to 40 mL of local anesthetic is required. When complete anesthesia of the leg is required, the combination of an anterior sciatic nerve block with femoral nerve block is a good alternative, as both blocks can be performed in the same sterile field without a change in position or repeated disinfection (Fig. 11.7 and Fig. 11.10).

The femoral nerve block should be performed before the sciatic nerve block. This allows a largely pain-free anterior proximal sciatic nerve block to be performed.

Catheter.

A catheter for continuous anesthesia or analgesia can be placed (Meier 1999a) because the sciatic nerve is surrounded by a fascial sheath from its emergence from the infrapiriform foramen until it enters the popliteal fossa (Clara 1959, Benninghoff and Goerttler 1975). Advancing the catheter more than 4 to 5 cm can lead to deviation of the catheter into the true pelvis (lesser pelvis) and should be avoided (see Fig. 11.24).

In a study by Meier (1999b) of 85 patients, the catheter was kept in situ for an average of 4 days (up to a maximum of 8 days). No infections were observed at the puncture site. No side effects, complications, or neurological deficits were found.

Continuous anterior sciatic nerve block.

Pain therapy through a continuous anterior sciatic nerve block can be provided with 6 mL/h of ropivacaine 0.33% (3.3 mg/mL) or bolus injections of 20 mL of ropivacaine 0.2 to 0.375% (2–3.75 mg/mL) every 6 to 8 hours (Meier 2001). The maximum dose of ropivacaine should not exceed 37.5 mg/h. Büttner and Meier (1999) reviewed continuous pain therapy with ropivacaine in clinical practice, which was performed without complications in over 6,000 peripheral catheters.

11.2.5 Anterior Proximal Sciatic Nerve Block Using Ultrasound

Curved array: 5 to 2 MHz, linear multifrequency broadband transducer with phased-array technique Penetration depth: 10 to 20 cm

Needle: 8 to 15 cm

Ultrasound Orientation

The sciatic nerve is visualized on the anterior proximal technique in supine position at the inside of the thigh 8 to 10 cm distal to the inguinal fold at the level of the lesser trochanter or slightly distal to it. The leg is abducted and rotated externally.

The following structures are sought:

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  The femur as a convex hyperechoic line with an acoustic shadow behind it

  
  
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  The femoral vessels (femoral artery and deep femoral artery)

Medial to and below the femoral arteries is the adductor muscle group, the largest muscle of which is the adductor magnus. The adductors originate at the femur. The sciatic nerve is in the layer between the adductor magnus and the gluteus maximus (Fig. 11.26). The sciatic nerve is hyperechoic here and has a round to oval shape (1.66 ± 0.68 cm maximum diameter) and can be visualized at a depth of 6.21 ± 0.68 cm below the surface of the skin (Chan et al 2006).

Related posts:

8 General Overview 14 Peripheral Nerve Blocks at the Ankle 10 Inguinal Paravascular Lumbar Plexus Anesthesia (Femoral Nerve Block) 12 Blocks at the Knee 9 Psoas Block 13 Peripheral Block (Conduction Block) of Individual Nerves of the Lower Limb